Semiconductor integrated circuits are the fundamental building blocks of modern electronic devices. Computers, cellular phones, and consumer electronics rely extensively on these devices, which may be used for storage of, computations on, and communication of data.
The most common semiconductor devices are formed using silicon as the primary substrate substance. Layers and regions of N-type material (such as elemental silicon), P-type material, and insulative material are combined to form electronic devices and circuits. N-type material is material which includes an excess of electrons. A typical method of producing N-type material is the introduction of certain atomic impurities into the semiconductor during growth of the semiconductor substrate. When certain other atomic impurities are introduced during growth, the resulting material will generally be P-type, having "holes", i.e., a deficit of electrons. In a P-type material, the holes act as charge carriers for the flow of electricity. In an N-type material, the excess electrons act as charge carriers. An insulator material is one which has a high resistance to current flow and may be used to isolate discrete components of a circuit, and act as a substrate on which active devices may be epitaxially grown.
The arrangement of P-type, N-type, and insulative materials and the respective electrical connections to each will determine what type of electrical device is created. Transistors, diodes, capacitors and most other electrical devices are created through the arrangement of these materials in a semiconductor device.
Recently, the advantages of using the Group III-V semiconductors (semiconductors formed from compound alloys including Group III and Group V elements) instead of silicon have led to extensive research and development. Among the typically used compounds and alloys are gallium arsenide (GaAs), aluminum gallium arsenide (AlGaAs) and indium phosphide (InP). The basic designs for the transistors and other devices used in silicon-based electronic devices have been adapted to Group III-V materials. Devices made from the Group III-V materials generally require lower power and are faster (operate at higher frequencies).
Group III-V semiconductor materials may also be used to produce optoelectronic devices, such as semiconductor lasers. In such devices an active region of un-doped or low-doped semiconductor material that is sandwiched between dual layers of P-type and N-type doped materials emits coherent light in response to the application of electrical current. The light is produced when holes from the P-type material recombine with electrons from the N-type material in the active region.
Other applications of the Group III-V materials are known to those in the art and include optical detectors, high-speed amplifiers and logic circuits. The widespread substitution of these semiconductors for silicon devices is impeded by relative difficulty and expense in producing group III-V semiconductors in comparison to the silicon devices.
One of the difficulties experienced with the use of InP is the process utilized to make InP semi-insulating layers. Pure InP would have a resistivity on the order of 10.sup.8 ohm-cm. However, in practice inP is obtainable only with impurities which significantly decrease the resistivity.
Iron doping has been adopted as a means for increasing the resistivity of indium phosphide. Typically, a precursor (the molecule containing the desired iron atom) of iron pentacarbonyl or ferrocene is used in conjunction with MOCVD growth of epitaxial indium phosphide. High resistivity on the order of 10.sup.9 ohm-cm is realized through this technique.
Such iron doping techniques have a number of difficulties. One of the difficulties is recognized by Dentai et al., U.S. Pat. No. 4,782,034. That patent noted that iron doped indium phosphide layers have poor thermal stability, i.e., performance is sensitive to temperature. Addressing this problem, the Dentai patent adopts doping using a titanium-based metal-organic dopant precursor. Similar to iron doping techniques, fairly high temperature is used in the growth to decompose the precursor according to Dental, on the order of 650.degree. C. Dentai contemplates decomposition of the titanium precursors at temperatures of up to 850.degree. C. Temperatures on this order may induce dopant diffusion which reduces the degree of control over the location of growth of the insulating material.
Frequently, it is desirable to selectively grow a semi-insulating material on a limited area which has been etched out of another layer. The reduction in control caused by the use of high temperatures may adversely affect this ability. At high temperatures the semi-insulating material has a tendency to cover the entire layer onto which it is being grown instead of only the desired etched-out areas. In other words, the doped indium phosphide grown at the higher temperatures may lay a blanket over the entire layer upon which it is being grown.
Another problem encountered in the use of iron doping techniques relates to the tendency of iron to remain reactive and migrate through subsequently grown layers. As subsequent layers are grown upon an iron doped indium phosphide layer acting as a substrate, for instance, the iron diffuses through the subsequently grown layers, and contaminates them.
Further difficulties may arise from the nature of the precursors used for iron doping techniques. The aforementioned ferrocene and iron pentacarbonyl leave behind a residue in the apparatus used to grow the InP. The sealed chamber used for iron-doped InP growth may actually have such residue absorbed in to the chamber walls. The residue then may act as a contaminant during further growth in the chamber. Thus, a separate crystal growth chamber system is sometimes dedicated to the growth of the iron-doped indium phosphide. This is expensive since the growth system may cost hundreds of thousands of dollars.
Indium phosphide is normally an N-type material. The effects of carbon tetrachloride doping to create P-type indium phosphide at conventional growth temperatures of 580.degree. and 630.degree. C. have been investigated. Cunningham et al, "Absence of .sup.13 C Incorporation in .sup.13 CCl.sub.4 --Doped InP Grown by Metalorganic Chemical Vapor Deposition", Applied Physics Letters 56 (18), pp. 1760-62, Apr. 30, 1990. Because carbon acts as an acceptor of electrons in other group III-V materials, such as gallium arsenide, Cunningham tested whether a carbon source like carbon tetrachloride could be used to produce P-type indium phosphide, the carbon accepting excess electrons present in the indium phosphide. The work by Cunningham found that doping with carbon at conventional growth temperatures of 580.degree. and 630.degree. resulted in no measurable change in carbon concentration and no change in carrier concentration. This suggests that the material grown at these conventional temperatures remained slightly N-type. The lack of carbon incorporation was attributed to the relatively weak bond formed between indium and carbon, the bond being broken at higher growth temperatures. The conclusion reached was that carbon was an impractical P-type dopant in indium phosphide and that extremely low growth temperatures below 450.degree. C. would be necessary to achieve any significant carbon incorporation. Later work of others suggested that carbon acts as a donor, or N-type dopant, in indium phosphide and would logically produce more severely N-type material.
However, at those temperatures below 450.degree., such as growth at 425.degree., carbon tetrachloride doped indium phosphide exhibits poor surface morphology. Morphology is the texture of the surface, which ideally should be mirror-like. Poor morphology makes it difficult to add successive layers of semiconductor material. In all, there is a need for methods which control the doping of InP to create high resistance in isolated regions of the substrate, without chamber contamination or poor surface morphology.
Accordingly, an object of the present invention is to provide an improved process for producing semi-insulating indium phosphide.
Another object of the invention is to provide an improved process for producing semi-insulating indium phosphide and other Group III-V compounds having resistivity exceeding approximately 10.sup.7 ohm-cm.
Yet another object of the invention is to provide an improved process for producing semi-insulating indium phosphide and other Group III-V compounds through use of precursors which will not contaminate a growth chamber in which the semiconductor is grown.
A further object of the invention is to provide an improved process for producing semi-insulating indium phosphide and other Group III-V compounds by using halide dopants, such as carbon tetrachloride and carbon tetrabromide at growth temperatures ranging from approximately 450.degree. C. to 525.degree. C.
A still further object of the invention is to provide an improved process for producing semi-insulating indium phosphide which allows practical incorporation of the indium phosphide into many semiconductor devices, for uses such as an insulating gate layer for metal-insulator-semiconductor field effect transistors (MISFETs), as a buffer layer to eliminate the effects of the Si impurity spike found at the epilayer/substrate interface on the pinch-off characteristics of InP field-effect transistors (FETs), as a current blocking layer in semiconductor lasers, as a device isolation layer in optoelectronic integrated circuits and as a Schottky-barrier enhancement layer for indium gallium arsenide based devices.